It is proposed to study the histogenesis of the lesion in buccal mucosa of the rat induced by 4 weeks on a severely zinc-deficient diet. Male rats of weanling age will be placed on a diet containing 0.8 ppm of zinc. Buccal mucosa from control rats on a zinc-adequate diet and experimental rats will be obtained after 1, 5, 9, 13, and 17 days. Zinc concentration will be assayed in the epithelium of the specimens isolated by microdissection. For qualitative evaluation and determination of the labelling index, mitotic index and subepithelial mast cell density, semi-thin sections will be used. They will be prepared from that portion of the specimen where the 4 week lesion is maximal. The aim of the study is to evaluate the carcinogenic potential of severe zinc deficiency by comparing the time sequence of changes in the mucosa to the known sequence of events following the implantation of a tramplantable tumor.